Since the Schottky electron gun and the field emission electron gun can stably emit an electric current of high brightness in a narrow energy spread, these electron guns are used for the electron gun of a charged particle beam device such as a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Particularly, these electron guns are used for the electron gun of an electron microscope for analysis because of the characteristics of a narrow energy spread and a high brightness, for example.
FIG. 3 schematically illustrates the configuration of a previously existing electron gun as a Schottky electron gun is taken as an example. The electron gun is configured of at least components below, including an electron source 1 formed of a tungsten single crystal material with a sharpened tip end, a filament 2 welded to the electron source 1 for heating the electron source 1, zirconium dioxide 3 coated over the electron source 1, a suppressor electrode 4 that suppresses thermoelectrons generated from the filament 2, an extracting electrode 5 that provides a strong electric field at the tip end of the electron source 1 for extracting electrons, and one or a plurality of accelerating electrodes 6 that accelerate the extracted electrons to a predetermined energy. The electron gun in FIG. 3 is the case of including one stage of the accelerating electrode. Moreover, the extracting electrode 5 includes an aperture 7 that restricts electrons (an electron beam) passed therethrough.
A negative potential V0 is applied to the electron source 1 with respect to the ground potential. When an electric current is passed through the filament 2, the filament 2 is heated at a temperature of about 1,800 K, and the zirconium dioxide 3 coated over the electron source 1 is diffused toward the tip end of the electron source 1. At this time, the work function on the tip end face of the electron source 1, that is, the work function on the crystal plane (100) of a single crystal is reduced to about 2.8 eV. Here, when a positive voltage V1 is applied to the extracting electrode 5 with respect to the electron source 1, the electric field near the tip end of the electron source 1 is increased, and electrons (an electron beam) are emitted from the crystal plane of the electron source 1, on which the work function is reduced, toward the extracting electrode 5 by Schottky effect (technically, electrons are emitted from crystal planes of tetragonal symmetry orthogonal to the crystal plane (100) such as the crystal plane (101) and the crystal plane (001) on the side faces near the tip end of the electron source in addition to the crystal plane (100) of the tip end of the electron source).
In the electrons emitted from the electron source 1, the electrons passed through the extracting electrode 5 are accelerated at a predetermined accelerating voltage by the accelerating electrode 6, and emitted from the electron gun. The electrons emitted from the electron gun are reduced to a specific magnification by a condenser lens and an objective lens, for example, not illustrated, and applied to a sample.
The electron microscope detects secondary electrons, transmission electrons, and reflection electrons generated by an interaction between electrons and a sample when the electrons collide against the sample, and observes and analyzes the microstructure of the sample.
Here, when an electron beam spot is observed through a fluorescent screen, for example, brightness called a flare is sometimes confirmed around a main beam.
FIG. 4 is a main beam 30 and a flare 31 of an electron beam spot actually observed. FIG. 4(a) is a photograph diagram and FIG. 4(b) is a schematic diagram. The flare 31 causes a reduction in signal-to-noise and a reduction in resolution of the observed image of the electron microscope and causes a system peak when analyzed.
As illustrated in FIG. 5, in Related Patent Document 1, it is considered that the flare is caused by electrons (an electron beam R) that an electron beam B2 emitted from crystal planes 1b (the crystal plane (010) and the crystal plane (001), for example) on the side faces near the tip end of an electron source (a tungsten single crystal) 1 is reflected in an extracting electrode 5. In this connection, a main beam B1 is emitted from a tip end face 1a of the electron source 1 (i.e. the crystal plane (100)). In Related Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 2008-117662), for the measures against the flare, a plurality of apertures 7 and 8 (two apertures, for example) were provided on an electron beam passage to geometrically restrict an angle at which the electrons are passed, as illustrated in FIG. 6. As a result, the reflection electron beam R from the extracting electrode caused by the electron beam emitted from the tip end side faces of the electron source 1 is geometrically restricted. Here, in the case where the apertures 7 and 8 are mounted on the extracting electrode 5, the apertures 7 and 8 restrict an angle of the electrons passed to an angle of 6°.